Lake Hylia

Do: Explore the lake
Get: Bottle, piece of heart, beginning of scarcrows song
Meet: Pierre, fishing dude

Swim up fastley, and you'll find yourself in lake hylia. Swim northwest of where land is, but only a little. Try to locate something underwater. If you are close enough, you can even L-target it to locate it easier. Once you collect it, navi will read what is inside:

Help me I'm waiting for you inside lord Jabu Jabu's belly.
-Princess Ruto
P.S. Don't tell my father.

Now you have the letter, you could exit, but you shouldn't want to, unlss you don't like secrets. Perhaps plant a magic bean on the right side of the land near the scince lab (a sign will say scince lab first) so you can get to the fishing hole as an adult before you beat water temple. From the scince lab, go forward until you see some scarecrows. Talk to the one closest to the water, then make up your own tune and eventually, when you're an adult, that will be the scarecrows song. From that place, swim over to the next small chunk of land near that. This is the fishing hole. Go inside and ask that fishing guy to fish. He will charge 20 rupees but who cares. Fish for the lunkers in the middle (check the secrets for info). Now go back to Zora's domain.